The present invention relates to a method for thermal treatment, especially drying, of finely comminuted bulk material as well as to an apparatus for carrying out this method and comprising a whirling fluidized bed reactor provided with an arrangement for feeding the material into the reactor means for feeding a whirling fluidizing gas into the reactor, a heating source connected to the reactor, and means for discharging the treated material and the fluidizing gas from the reactor.
During thermal treatment, especially drying, of finely particulated bulk material, such as pulverulent or dust-like material, as used in the chemical, pharmaceutical or foodstuffs industry, the uniformity of the treatment and the prevention of agglomeration of the material is of extreme importance.
It is known to dry such finely particulated material in contact driers. In a contact drier described in the book "Das Trocknen" by F. Kneule, published 1975 by Sauerlander, the material to be dried is transported by means of a screw conveyor while the heat necessary for the drying is indirectly transmitted to the material by means of a jacket heater. However, in this construction the powder is liable to form at the wall of the screw conveyor an insulating layer which prevents passing of the heat through the material. This requires a relatively high heat input and higher drying temperatures, which in turn may lead to the formation of heat nests. In a stream drier, described on pages 355-370 of the above mentioned book, the transport of the material is carried out by means of a heated air stream which is guided through a system of channels. Thereby, a direct heat exchange between the heated air and the powder is accomplished. To separate the powder from the transporting air, a cyclone is arranged downstream of the drier. In this drying arrangement, the time the powder remains in the drying phase is very small. This, in turn, requires a high temperature of the drying air, so that also in this method a relatively high heat requirement exists. In addition, the energy requirement for the transport of air and material to be dried is considerable. In a whirling drier, as described in the aforementioned book on pages 331-335, and further described in a book "Trockner und Trocknungsverfahren" by K. Kroell, published by Springer, 2nd edition, 1978, pages 238-246, a fluidized bed is produced by means of a heated air stream passing through an opening in the bottom into the material to be dried. The heat exchange occurs in this case directly between the heated air and the powder. For separating the heated air from the powder, a cyclone or filter is arranged downstream of the drying arrangement. In such a whirling layer drier the danger of locally overheating the material does not exist; however, the results of this last mentioned drier are not very satisfactory because the drying often occurs not uniformly.